Conventionally, an electrophotographic image forming apparatus has been widely used as an image forming apparatus such as a copying machine, a printer, or a facsimile machine.
In the electrophotographic image forming apparatus, the following processes are made. Specifically, a photoreceptor surface of an image holding member such as a photoreceptor drum is charged by a charging device, and the photoreceptor surface thus charged is exposed to light by an exposure device. As a result, an electrostatic latent image is formed on the photoreceptor surface. Then, the electrostatic latent image thus formed is developed by a developing device, thereby forming a toner image (visible image) on the photoreceptor surface. Thereafter, the toner image is transferred onto a sheet (recording medium, such as plain paper or OHP recording paper) by a transfer device and then fixed to the sheet by a fixing device.
Meanwhile, in recent years, toner has been downsized to improve image quality or increase a printing speed. This results in frequent scattering and floating of such toner, in the image forming apparatus. Especially, floating toner contained in the air exhausted from the image forming apparatus has become a problem.
In view of the circumstances, Japanese Unexamined Patent Publication No. 272122/2004 (published on Sep. 30, 2004) teaches a technique in which a vacuum generator for sucking air via an air filter is provided to a developing device. The vacuum generator sucks floating toner existing in the vicinity of an aperture of the developing device.
Further, Japanese Unexamined Patent Publication No. 003220/1998 (Tokukaihei 10-003220, published on Jan. 6, 1998) teaches a technique to provide airflow generating means for generating an airflow, from the outside to the inside of the development apparatus, in a gap between (i) a latent-image holding member and (ii) carrier collecting means, a part of which is exposed through an opening of a casing of a development unit.
Further, the electrophotographic image forming apparatus has another problem. Specifically, ozone or nitrogen oxide is generated in the charging device and contaminates the environment around the image forming apparatus. Furthermore, it is known that if nitrogen oxide adheres to a photoreceptor drum, an image defect called whiteout occurs.
In order to prevent generation of ozone or nitrogen oxide in a charging device, Japanese Unexamined Patent Publication No. 38728/1999 (Tokukaihei 11-38728, published on Feb. 12, 1999) teaches a technique to close the space between a charging member and a member to be charged in an image forming apparatus adopting a charging device of contact electrification. By reducing the pressure of the closed space, charging efficiency can be improved.
Further, Japanese Unexamined Patent Publication No. 241484/2003, teaches a charging device including (i) a corona-charger with a conductive shield and (ii) a duct provided surrounding the corona-charger, wherein, during rotation of an image holding member, A>B>C, where A is an air-pressure in the shield, B is an air-pressure in the duct, and C is an air-pressure outside of the duct. This prevents a discharge product, such as ozone, NOx, and the like, from adhering to the image holding member.
The above techniques, however, have a problem that if humidity within the image forming apparatus changes, image quality is degraded and -processing becomes less efficient.
For example, with the technique of Japanese Unexamined Patent Publication No. 2004-272122, it is possible to reduce the amount of scattered toner, but the humidity in the space between the photoreceptor and the developing device becomes substantially the same as the circumambient humidity. Therefore, the increase in the circumambient humidity causes a reduction in a resistance of toner (developer), so that the amount of charges decreases. This degrades the performance of development.
Further, since the technique of Japanese Unexamined Patent Publication No. 38728/1999 (Tokukaihei 11-38728) is applicable to a charging device of contact electrification, the technique is not applicable to a charging device of non-contact electrification such as corona charging. In a charging device of corona charging, if the humidity increases in the space between the charging member (discharge electrode) and the photoreceptor, a discharge onset voltage also increases. This reduces charging efficiency. Therefore, according to the technique of Japanese Unexamined Patent Publication No. 38728/1999 (Tokukaihei 11-38728), it is not possible to prevent a reduction in charging efficiency due to a change in humidity, in the charging device of non-contact electrification. It is well known from Paschen's law that a discharge onset voltage is in proportion to a product of gas pressure and an interval between charging electrodes.
Further, in the transfer device, resistances of respective sections change in response to a change in humidity of, for example, a transfer member, a sheet transporting path (sheet path), a sheet, and/or toner, and therefore transfer efficiency decreases.
Further, in the fixing device, humidity change causes a change in each heat conductivity of, for example, members of the fixing device, a sheet, or toner to change. This causes heat energy to be used less efficiently, and therefore fixing efficiency decreases.